You can read the Demon Slayer manga online at these online websites. You can also purchase the volumes you need on these websites.
Read Kimetsu No Yaiba Chapter 6 : Pile O' Hands on Mangakakalot
Demon Slayer Vol. 1 Review – KidzNet
Demon Slayer Has Slowly but Surely Worn Out Its Welcome
Demon Slayer Vol. 1 Review – KidzNet
If I watched the demon slayer anime, then from where should I start reading manga? How many chapters are there? - Quora
VIZ Read a Free Preview of Demon Slayer: Kimetsu no Yaiba, Vol. 1
VIZ Read Demon Slayer: Kimetsu no Yaiba Manga Free - Official Shonen Jump From Japan
VIZ Read a Free Preview of Demon Slayer: Kimetsu no Yaiba, Vol. 22
Here's Where to Start the Demon Slayer Manga After Season 3 Finale